Tire containing improved liner

ABSTRACT

Disclosed are crosslinking compositions having both sulfur and nonsulfur curative systems which are particularly adapted for crosslinking rubber blends of (a) from about 35% to about 85% halobutyl rubber or a mixture of butyl rubber and halobutyl rubber wherein the halobutyl rubber is at least 50% by weight of the mixture and (b) from about 15% to about 65% of an epihalohydrin rubber, and the rubber materials crosslinked therewith. The rubber materials crosslinked with these crosslinking compositions have an excellent balance of cure rate, cure state and scorch safety; have improved tire carcass adhesion and good impermeability to gases; and are useful in tire inner tubes and inner liners.

This application is a division, of application Ser. No. 691,334, filed Jan. 14, 1985.

This invention relates to a crosslinking composition having sulfur and nonsulfur curative systems, said composition being adapted particularly for crosslinking blends of (a) halobutyl rubber or a mixture of butyl rubber and halobutyl rubber wherein the halobutyl rubber is at least 50% by weight of the mixture and (b) an epihalohydrin rubber, and to the rubber materials crosslinked therewith. The rubber materials cured with the crosslinking composition of this invention have an excellent balance of cure rate, cure state, scorch safety and physical properties; have improved tire carcass adhesion and good impermeability to gases, especially air; and are useful in tire inner tubes and inner liners for pneumatic tires, where sensitivity to gas entrapment is important.

Tire liners based on blends of chlorobutyl rubber with natural rubber or reclaimed butyl rubber or both, experience difficulty due to sensitivity to entrapment of gases, particularly air. In the past, epichlorohydrin rubber has been added to the aforementioned blends to improve impermeability (See, e.g., U.S. Pat. No. 3,586,087). However, while these tire liner compositions have good air impermeability, they do not have a good balance of cure rate, scorch safety and physical properties, and do not adhere well to the tire carcasses, which are primarily natural rubber.

This invention provides a novel crosslinking composition for use in crosslinking or vulcanizing rubber blends of, by weight, (a) from about 35% to about 85% of halobutyl rubber or a mixture of butyl rubber and halobutyl rubber wherein the halobutyl rubber is at least 50% by weight of the mixture, and (b) from about 15% to about 65% of epihalohydrin rubber. The crosslinking composition consists essentially of, by weight of the rubber blend, (1) from about 1.0 parts to about 20 parts of a sulfur curative system, as hereinafter defined, and (2) from about 0.3 parts to about 10 parts of a nonsulfur curative system, as hereinafter defined.

The crosslinking compositions of this invention provide cured or crosslinked rubber materials have good impermeability, i.e., resistance to gas or air diffusion, without an adverse effect on the physical properties of the cured composition, such as tensile strength and modulus.

The crosslinking composition is formulated with (1) a sulfur cure package comprising sulfur, a sulfur accelerator, and a zinc oxide promotor and (2) a nonsulfur curative system comprising a nonsulfur crosslinking agent and a metal compound promotor. Sulfur curative systems cure through the unsaturation present in halobutyl rubber or mixtures thereof with butyl rubber. The nonsulfur curative systems cure through the halogen functionally of the halogen-containing rubbers of the blend.

Suitable sulfur accelerators include mercaptobenzothiazole and its derivatives, such as mercaptobenzothiazole disulfide, 2,2'-dithiobis(benzothiazole) and its salts; sulfenamides; thiurams, such as tetramethylthiuram; and dithiocarbamate salts, such as cadmium diethyl dithiocarbamate. Typical sulfenamides include N-cyclohexyl-2-benzothiazyl-sulfenamide, and N,N-diisopropyl-2-benzothiazyl-sulfenamide.

Suitable nonsulfur curatives include di- and trifunctional mercapto compounds and their derivatives, such as 2,5-dimercapto-1,3,4-thiadiazole, 2-mercapto-1,3,4-thiadiazole-5-benzoate, trithiocyanuric acid, dithiohydantoins, dimercaptotriazoles, dithioltriazene, and their ester derivatives, alone or together with an appropriate organic base activator as set forth in U.S. Pat. Nos. 4,128,510 and 4,288,576.

Oxides, hydroxides and carbonates of the metals in Groups Ia and IIa of the Periodic Table of the Elements, such as magnesium oxide and barium carbonate, are typical metal compound promotors which can be used to promote the cure by the nonsulfur curative component of the crosslinking compositions of this invention.

The optimum amounts of the crosslinking compositions will depend upon the degree of crosslinking desired, the degree of scorch safety desired, and the rate of cure desired. The determination of the amounts to be used is well within the skill of those in the art.

The sulfur curative system is present in the crosslinking compositions of this invention in an amount, by weight of the rubber blend, from about 1.0 parts to about 20 parts, preferably from about 1.7 parts to about 9 parts, and most preferably about 3.0 parts to about 7 parts. The nonsulfur curative system usually is present in the compositions of this invention in an amount from about 0.3 parts to about 10 parts, preferably from about 0.5 parts to about 7 parts, and most preferably 0.7 parts to about 5 parts.

The sulfur curative system consists essentially of, by weight of the crosslinking composition, from about 0.1 parts to about 5 parts sulfur, preferably from about 0.2 parts to about 1 part; from about 0.1 parts to about 5 parts of a sulfur accelerator, preferably from about 0.5 parts to about 3 parts; and from about 0.5 parts to about 10 parts of a zinc oxide promotor, preferably from about 1.0 parts to about 5 parts, and most preferably from about 1.5 parts to about 3.0 parts.

Typically the nonsulfur curative system contains, by weight of the crosslinking system, from about 1.0 parts to about 5 parts of a nonsulfur curative compound, preferably from about 0.2 parts to about 2 parts, and from about 0.2 parts to about 10 parts of a Group Ia or IIa metal compound promotor, preferably from about 0.3 parts to about 5 parts, and most preferably from about 0.5 parts to about 3.0 parts. The Group Ia or IIa metal compound promotor also functions as an acid acceptor.

The rubber blends can be prepared by blending the rubber compounds together with all desired ingredients, except the crosslinking composition of this invention, in any desired fashion that allows intimate mixing; for example, in a Banbury mixer or two roll differential speed mill.

The crosslinking composition of this invention can then be incorporated or mixed with the blended ingredients in any desired fashion, such as by simply milling the blended ingredients and the crosslinking composition on a conventional rubber mill. Preferably the milling is conducted at 50° C. to about 90° C. Other method of mixing the crosslinking composition with the polymer will be apparent to those skilled in the art.

Crosslinking is effected at elevated temperatures. In general the crosslinking temperature will be from about 140° C. to about 260° C., preferably from about 150° C. to about 225° C., and most preferably from about 150° C. to about 205° C. The time will vary inversely with the temperature and will range from about 5 seconds to 10 hours. The crosslinking temperatures and times are within the skill of those versed in the art.

As is well known in the art, butyl rubber is a copolymer of from about 95.5 to about 99.5 mole % isobutylene and from about 0.5 to about 4.5 mole % isoprene.

Halobutyl rubbers are halogenated butyl rubbers. The halogen is present as a result of post-treatment of the butyl rubber by reacting chlorine or bromine with the butyl rubber by methods known in the art. Halobutyl rubbers include chlorobutyl, bromobutyl or mixtures of chloro and bromobutyl rubbers.

Such materials include chlorobutyl and bromobutyl rubbers prepared by reacting chlorine or bromine with butyl rubber wherein the original unsaturation (usually from about 0.5 to 3.0 mole % isoprene residues) is largely maintained. A suitable commercially available chlorobutyl rubber contains from about 1.1% to about 1.35% by weight of chlorine from about 0.5 to 2 mole % unsaturation, and has a Mooney viscosity after 8 minutes at 212° F. using the large (4-inch) rotor of 40 to 89 ML. A satisfactory bromobutyl rubber prepared from a highly unsaturated butyl rubber contains from 2.1 to 3.0% by weight of bromine, a density at 25° C. of about 0.96, and evidences a Mooney viscosity at 212° F. of 40 to 70 ML.

Generally, the halobutyl rubber or the mixture of butyl rubber and halobutyl rubber is present in an amount of from about 35% to about 85% by weight of the rubber content, preferably from about 40% to about 75%. Halobutyl rubber is preferred.

Epihalohydrin rubber includes (1) homopolymers of an epihalohydrin, such as epichlorohydrin, epibromohydrin and others, (2) copolymers of an epihalohydrin with less than 30% of saturated epoxy monomers or with an unsaturated epoxy monomer, and (3) terpolymers of an epihalohydrin with (a) less than 30% of a saturated epoxy monomer or mixtures thereof, (b) an unsaturated epoxy monomer or mixtures thereof, or (c) mixtures of (a) and (b). The epihalohydrin polymers are prepared by polymerizing a monomeric epihalohydrin alone or together with one or more of the aforementioned epoxy monomers with a suitable catalyst, such as an organometallic catalyst. For example, a reaction product of water with an alkyl aluminum compound is a suitable organometallic catalyst. Such homopolymers are believed to have a repeating structure: ##STR1## wherein X is a halogen, derived by a polymerization involving the epoxide linkages. Suitable halogens include chlorine and bromine. Typical saturated epoxy monomers include alkylene oxides, such as ethylene oxide, and typical unsaturated epoxy monomers include allylglycidyl ether.

The epihalohydrin polymers are strong, rubbery materials sometimes evidencing more or less crystallinity, having high molecular weight, a density in the range of from about 1.35 to about 1.38, and after 4 min. at 212° F., a Mooney viscosity of from about 40 to about 80 ML. The properties and the preparation of epihalohydrin polymers suitable for use in the practice of this invention are more fully disclosed in U.S. Pat. No. 3,158,500, the disclosure of which is incorporated herein by reference.

Copolymers and terpolymers of an epihalohydrin with greater than 30% of saturated epoxy monomers, especially with alkylene oxide monomers, such as ethylene oxide, do not have the required air impermeability and are not useful in tire inner liners and inner tubes.

Copolymers or terpolymers of epihalohydrin with unsaturated epoxy monomers, such as allylglycidyl ether, tend to cocure with the unsaturation of the halobutyl rubber or mixtures thereof in the rubber blend and the natural rubber in the carcass. Preferably, from about 1% to about 30% of an unsaturated epoxy monomer is used. Copolymers and terpolymers of epihalohydrin with a saturated epoxy monomer preferably contain less than about 10% of the saturated epoxy monomer, and most preferably less than about 5%.

Preferably the epihalohydrin rubber is present in an amount of from about 15% to about 65% by weight of the total rubber blend, preferably from about 25% to about 55%.

The amount of (a) halobutyl rubber or mixtures thereof with butyl rubber and (b) epichlorohydrin rubber present in the rubber blend totals 100%.

In addition to the crosslinking composition of this invention, the rubber blend may contain other ingredients commonly used in rubber vulcanization. For example, extenders, fillers, pigments, stabilizers, antioxidants, tackifiers, plasticizers, and softeners may be added. The presence of a filler, particularly carbon black, while not necessary, is beneficial. The particle size of the filler is not critical.

Other additives such as polychloroprene rubber, mineral rubber (blown asphalt), and chlorinated hydrocarbon polymers, such as chlorinated polyisoprene rubber and chlorinated polyolefins, may be present in an amount from about 1% to about 10% by weight of the rubber blend.

Typically the chlorinated hydrocarbon polymers have a chlorine content of from about 30% to about 70%, by weight, preferably 40% to about 66%, and a molecular weight of about 50,000 to about 1,000,000 as determined by size exclusion chromatography. Chlorinated polyisoprene having from about 63% to about 66% by weight chlorine is the preferred chlorinated hydrocarbon polymer.

The polychloroprene improves green strength, the mineral rubber improves aged adhesion, and the chlorinated hydrocarbon polymers generally tend to improve the impermeability to air and other gases.

The following examples set forth in Table 1 illustrate the invention. The controls or comparative examples are set forth in Table 2. The examples and the controls are prepared by the same procedure as set forth above. The properties of the examples and controls are set forth in Tables 3 and 4, respectively.

                                      TABLE 1                                      __________________________________________________________________________                 Examples, Parts by Wt.                                             Ingredients 1   2   3   4   5   6  7  8  9  10 11 12 13 14 15                  __________________________________________________________________________     Chlorobutyl rubber.sup.(a)                                                                 75  75  75  75  60  50 75 75 75 75 -- -- -- 37.5                                                                              75                  Bromobutyl rubber.sup.(b)                                                                  --  --  --  --  --  -- -- -- -- -- 75 75 37.5                                                                              -- --                  Butyl rubber.sup.(c)                                                                       --  --  --  --  --  -- -- -- -- -- -- -- 37.5                                                                              37.5                                                                              25                  Polyepichlorohydrin                                                                        25  25  25  25  40  50 25 25 25 25 25 25 25 25 --                  Carbon black N-990.sup.(d)                                                                 50  --  50  --  --  -- -- -- -- -- -- -- -- -- 50                  Carbon black N-762.sup.(e)                                                                 --  50  --  50  50  50 50 50 50 50 50 50 50 50 --                  Stearic acid                                                                               1.0 1.0 1.0 1.0 1.0 1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                 Alkyl phenolic resin,                                                                      3.3 3.3 3.3 3.3 3.3 3.3                                                                               3.3                                                                               3.3                                                                               3.3                                                                               3.3                                                                               3.3                                                                               3.3                                                                               3.3                                                                               3.3                                                                               3.3                 Ring & Ball s. pt.                                                             of 90° C.                                                               Chlorinated polyiso-                                                                       --  --  --  --  --  -- -- -- -- -- -- -- -- -- 10                  prene rubber                                                                   (˜65% chlorine)                                                          Paraffin oil.sup.(f)                                                                       8.3 --  8.3 --  --  -- -- -- -- -- -- -- -- -- --                  Nickel dibutyl dithio-                                                                     1.0 1.0 --  --  1.0 1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                                                                               1.0                 carbamate                                                                      Magnesium oxide                                                                            0.625                                                                              0.625                                                                              0.625                                                                              0.625                                                                              1.25                                                                               1.25                                                                              0.62                                                                              0.25                                                                              3.0                                                                               0.62                                                                              1.25                                                                              1.25                                                                              1.0                                                                               0.25                                                                              0.625               Zinc oxide (ZnO)                                                                           2.25                                                                               2.25                                                                               2.25                                                                               2.25                                                                               1.75                                                                               1.5                                                                               1.0                                                                               2.25                                                                              2.25                                                                              4.0                                                                               2.25                                                                              2.25                                                                              3.0                                                                               3.0                                                                               2.25                Mercaptobenzothia-                                                                         2.0 2.0 2.0 2.0 0.825                                                                              0.75                                                                              1.13                                                                              1.13                                                                              1.13                                                                              3.0                                                                               3.0                                                                               3.0                                                                               2.0                                                                               2.0                                                                               2.0                 diazole disulfide                                                              Sulfur      0.375                                                                              0.375                                                                              0.375                                                                              0.375                                                                              0.275                                                                              0.25                                                                              0.2                                                                               0.38                                                                              0.38                                                                              0.6                                                                               0.4                                                                               0.4                                                                               0.37                                                                              0.37                                                                              0.375               Ethylene thiourea                                                                          --  --  --  --  --  -- -- -- -- -- -- -- -- -- --                  2-Mercapto-1,3,4-                                                                          0.7 0.7 0.7 0.7 1.25                                                                               1.4                                                                               0.7                                                                               0.25                                                                              0.25                                                                              0.7                                                                               0.3                                                                               0.7                                                                               0.2                                                                               0.3                                                                               0.7                 thiadiazole-5-                                                                 benzoate.sup.(g)                                                               Tetramethylthiuram                                                                         --  --  --  --  --  -- -- -- -- -- -- -- 0.2                                                                               0.37                                                                              --                  disulfide                                                                      __________________________________________________________________________      .sup.(a) Chlorinated isopreneisobutylene copolymer, mole % unsaturation        ˜1.7%; ˜1.2% chlorine content; and 55 Mooney viscosity at          100° C.                                                                 .sup.(b) Brominated isopreneisobutylene copolymer, mole % unsaturation         ˜1.6%; ˜2.1% bromine content and 40 Mooney viscosity at            100° C.                                                                 .sup.(c) Isopreneisobutylene copolymer, mole % unsaturation ˜1.4%        and 45 Mooney viscosity at 100° C.                                      .sup.(d) ASTM D1510, I.sub.2 absorption = 7.                                   .sup.(e) ASTM D1510, I.sub.2 absorption = 26.                                  .sup.(f) 2640 SUS viscosity at 100° F.                                  .sup.(g) Sixtysix percent (66%) active on clay.                          

                  TABLE 2                                                          ______________________________________                                                        Controls, Parts by Wt.                                          Ingredients     1      2      3     4    5                                     ______________________________________                                         Chlorobutyl rubber.sup.(a)                                                                     75     75     25    75   75                                    Bromobutyl rubber.sup.(b)                                                                      --     --     --    --   --                                    Butyl rubber.sup.(c)                                                                           --     --     --    --   --                                    Polyepichlorohydrin                                                                            25     25     75    25   25                                    Carbon black N-990.sup.(d)                                                                     50     --     --    --   --                                    Carbon black N-762.sup.(e)                                                                     --     50     50    50   50                                    Stearic acid    1.0    1.0    1.0   1.0  1.0                                   Alkyl phenolic resin,                                                                          3.3    3.3    3.3   3.3  3.3                                   Ring & Ball s. pt.                                                             of 90° C.                                                               Chlorinated polyiso-                                                                           --     --     --    --   --                                    prene rubber                                                                   (˜65% chlorine)                                                          Paraffin oil.sup.(f)                                                                           8.3    --     --    --   --                                    Nickel dibutyl dithio-                                                                         --     --     1.0   --   --                                    carbamate                                                                      Magnesium oxide 2.25   2.25   1.88  --   1.25                                  Zinc oxide (ZnO)                                                                               --     --     0.75  3.0  --                                    Mercaptobenzothiadiazole                                                                       --     --     0.375 1.5  --                                    disulfide                                                                      Sulfur          --     --     0.12  0.5  --                                    Ethylene thiourea                                                                              0.75   0.75   --    --   --                                    2-Mercapto-1,3,4-                                                                              --     --     2.1   --   3.0                                   thiadiazole-5-                                                                 benzoate.sup.(g)                                                               Tetramethylthiuram                                                                             --     --     --    --   --                                    disulfide                                                                      ______________________________________                                          .sup.(a) Chlorinated isopreneisobutylene copolymer, mole % unsaturation        ˜1.7%; ˜1.2% chlorine content; and 55 Mooney viscosity at          100° C.                                                                 .sup.(b) Brominated isopreneisobutylene copolymer, mole % unsaturation         ˜1.6%; ˜2.1% bromine content and 40 Mooney viscosity at            100° C.                                                                 .sup.(c) Isopreneisobutylene copolymer, mole % unsaturation ˜1.4%        and 45 Mooney viscosity at 100° C.                                      .sup.(d) ASTM D1510, I.sub.2 absorption = 7.                                   .sup.(e) ASTM D1510, I.sub.2 absorption = 26.                                  .sup.(f) 2640 SUS viscosity at 100° F.                                  .sup.(g) Sixtysix percent (66%) active on clay.                          

    TABLE 3       Examples, Parts by Wt. Properties 1 2 3 4 5 6 7 8 9 10 11 12 13 14      15      Adhesion to Tire Carcass.sup.(a)       T-Peel lbs/in-pli 58 54 38 42 48 37 40 38 53 58 55 48 45 32 52   ODR      Cure at 171° C..sup.(b) Minimum, in-lbs 5.0 10.5 5.5 10.5 14 15.5      11.6 11.9 12.5 10.5 9.6 10.2 10.0 11.0 12.0  Type MHF.sup.(f) MHF MHF      MHF MHF MHF MHF MHF MHF MHF MHF MHF MH.sup.(g) MH MHF  Maximum, in-lbs      22.2 38.0 19.5 36 40 42 30 33 38 40 38 40 30 33 42  Ts2, min. 2.6 2.2      2.5 2.2 2.1 2.0 3.0 2.5 2.0 2.5 2.5 2.0 3.5 3.75 2.3  T.sub.c (90), min      6.4 8.25 6.0 8.5 9.0 9.0 15.5 14.0 11.0 10.5 10.5 8.0 16.5 18.0 8.0      Mooney Scorch @ 121° C..sup.(c) Minimum viscosity 14 27.0 13.5      26.5 27.5 28.0 27.0 26.0 27.0 26.0 24.0 26.5 23.5 25.5 28.5  Time to 3      pt. rise, min. 30 23.5 >30 26.5 20.5 21.0 28.5 24.0 18.5 19.0 28.0 16.75      24.5 23.0 21.0  Time to 5 pt. rise, min. >30 31 >30 >30 26.5 26.0 >30      >30 25.0 28.75 >30 22.5 28.0 26.5 28.5  Physical Properties.sup.(d) 100%      Modulus, psi 110 230 110 230 245 255 200 210 230 280 260 280 240 245 250       200% Modulus, psi 150 400 150 400 410 410 390 400 450 520 430 400 410      400 440  300% Modulus, psi 200 520 200 520 520 540 480 490 540 720 550      572 540 530 560  Tensile strength, psi 920 1300 940 1350 1300 1300 960      1080 1170 1200 1360 1420 1580 1620 1100  % Elongation 760 770 740 760      770 780 780 720 640 500 740 670 750 730 690  Shore A Hardness 47 60 50      62 63 64 60 60 62 67 65 68 64 62 65       Air Permeability at 66.7 C.sup.(e)4.23.04.23.12.92.73.1 3.13.02.93.02.93      .13.12.8 10.sup.-10 cm.sup.2 cm cm.sup.3 sec cm Hg      .sup.(a) Cured adhesion is measured by placing together a sheet (80 mil      thick) of the uncured tire carcass compound with crosslinkers added and a      sheet (80 mil) of uncured rubber blends containing the crosslinking      compositions of this invention. A polyester film is placed between the      sheets at one end to provide a tab for pulling. Each sheet is backed with      a cloth sheet. The pad is placed in a mold about 160 mil deep. The pad is      cured for 15 minutes at 182° C. Oneinch wide strips are cut and th      peel adhesion is determined by measuring the force to pull apart the cure      strips according to the procedures of ASTM D41838. The average of three      pulls is reported.      .sup.(b) Osillating disc rheometer ASTM D2084-71-T.      .sup.(c) ASTMD-1646.      .sup.(d) ASTM D412.      .sup.(e) For vulcanizates cured at 160° C. for 30 min.      .sup.(f) Equilibrium torque.      .sup.(g) Highest value of torque where no constant or maximum is obtained

                  TABLE 4                                                          ______________________________________                                                      Controls, Parts by Wt.                                            Properties     1       2      3     4     5                                    ______________________________________                                          Adhesion to Tire Carcass.sup.(a)                                              T-Peel lbs/in-pli                                                                             35      45     15    42    34                                   ODR Cure at 171° C..sup.(b)                                             Minimum, in-lbs                                                                               5.5     10     17.5  10.5  10.5                                 Type           MH.sup.(g)                                                                             MH     MHF.sup.(f)                                                                          MHF   MH                                   Maximum, in-lbs                                                                               11.0    21.0   48    35.0  32.8                                 Ts2, min.      7.5     3.8    1.8   2.0   2.0                                  T.sub.c (90), min                                                                             34      37     12.6  17.5  25.5                                 Mooney Scorch                                                                  @ 121° C..sup.(c)                                                       Minimum viscosity                                                                             15      27.5   32.0  27.0  27.0                                 Time to 3 pt. rise, min.                                                                      19.5    14.5   12.0  12.0  10.5                                 Time to 5 pt. rise, min.                                                                      23.5    16.75  16.5  14.5  12.75                                Physical Properties.sup.(d)                                                    100% Modulus, psi                                                                             90      200    270   140   230                                  200% Modulus, psi                                                                             120     310    460   270   360                                  300% Modulus, psi                                                                             140     410    585   410   470                                  Tensile strength, psi                                                                         340     780    1280  1310  700                                  % Elongation   1030    710    820   830   840                                  Shore A Hardness                                                                              43      57     66    57    57                                   Air Permeability                                                                              4.5     3.2    2.4   3.2   3.0                                  at 66.7 C.sup.(e)                                                              10.sup.-10 cm.sup.2 cm                                                         cm.sup.3  sec cm Hg                                                            ______________________________________                                          .sup.(a) Cured adhesion is measured by placing together a sheet (80 mil        thick) of the uncured tire carcass compound with crosslinkers added and a      sheet (80 mil) of uncured rubber blends containing the crosslinking            compositions of this invention. A polyester film is placed between the         sheets at one end to provide a tab for pulling. Each sheet is backed with      a cloth sheet. The pad is placed in a mold about 160 mil deep. The pad is      cured for 15 minutes at 182° C. Oneinch wide strips are cut and th      peel adhesion is determined by measuring the force to pull apart the cure      strips according to the procedures of ASTM D41838. The average of three        pulls is reported.                                                             .sup.(b) Osillating disc rheometer ASTM D2084-71-T.                            .sup.(c) ASTMD-1646.                                                           .sup.(d) ASTM D412.                                                            .sup.(e) For vulcanizates cured at 160° C. for 30 min.                  .sup.(f) Equilibrium torque.                                                   .sup.(g) Highest value of torque where no constant or maximum is obtained                                                                               

Adhesion to the tire carcass of at least 25 T-Peel lbs./in.-pli is acceptable; however, preferably it is greater than 30 T-Peel lbs./in.-pli.

A Mooney scorch of 16 minutes or better is good for the 3 point rise, and 20 minutes or better is good for the 5 point rise.

Tensile strength of at least 900 psi is acceptable, preferably greater than 900 psi.

Control 1 shows that a rubber blend containing a large particle size carbon black and oil when crosslinked with magnesium oxide and ethylene thiourea conventional crosslinking system does not provide an acceptable tensile strength, whereas the same rubber-based composition (Example 1) when it is cured with the crosslinking composition of this invention does provide acceptable tensile strength.

Control 2 shows that the same rubber blend containing small particle size carbon black and no oil when crosslinked with a conventional crosslinking system likewise does not provide an acceptable tensile strength or scorch safety, whereas the same composition (Example 2) when it is cured with the crosslinking composition of this invention does provide acceptable tensile strength and scorch safety.

Control 3 shows that a rubber blend containing smaller amounts of chlorobutyl rubber (25%) when crosslinked with the crosslinking system of this invention does not provide a material having acceptable scorch or adhesion properties.

Control 4 shows that the sulfur curative system alone and Control 5 shows that the nonsulfur curative system alone, when used to crosslink a representative rubber blend, do not provide materials having acceptable scorch properties. Further, nonsulfur curative system alone (Control 5), does not provide a material having acceptable tensile strength properties.

Usually oil makes rubber compositions more permeable to gases. While the composition of Example 1 is more permeable to gases than the composition of Example 2, it still has improved scorch safety, cure rate and physical properties.

The tire carcass is typically natural rubber with other rubbers, such as styrene-butadiene, added in lesser amounts and cured with a conventional sulfur cure system.

Thus, this invention provides a sulfur and nonsulfur crosslinking composition especially adapted for crosslinking blends of (a) halobutyl rubber or a mixture of butyl rubber and halobutyl rubber wherein the halobutyl rubber is at least 50% by weight of the mixture; and (b) an epihalohydrin rubber. Rubber blends cured with the compositions of this invention have a better balance of cure rate, cure state, scorch safety and physical properties and have improved adhesion to the tire carcasses and good impermeability to gases. The cured rubber material is especially useful in tire inner tubes and tire inner liners, particularly in a pneumatic tire having cured rubber liner ply overlying all internal tire surfaces exposed in service to pressurized gas.

The expressions "consists essentially of" and "consisting essentially of" as used in this specification excludes any unrecited substance at a concentration sufficient to substantially adversely affect the essential properties and characteristics of the composition of matter being defined, while permitting the presence of one or more unrecited substances at concentrations insufficient to substantially adversely affect said essential properties and characteristics.

Features, advantages and other specific embodiments of this invention will become readily apparent to those exercising ordinary skill in the art after reading the foregoing disclosures. In this regard, while specific embodiments of this invention have been described in considerable detail, variations and modifications of these embodiments can be effected without departing from the spirit and scope of the invention as disclosed and claimed. 

What is claimed is:
 1. In a pneumatic tire having cured rubber liner ply overlying all internal tire surfaces exposed in service to pressurized gas and having, as essential rubbery constituents, (a) (i) halobutyl rubber or (ii) a mixture of butyl rubber and halobutyl rubber wherein the halobutyl rubber is present in an amount of at least 50% by weight of the mixture; and (b) an epihalohydrin rubber, the improvement which comprises curing the rubber liner with a composition consisting essentially of, for each 100 parts by weight of the rubber blend: (a) from about 1.0 part to about 20 parts of a sulfur curative system consisting essentially of (i) sulfur, (ii) an accelerator, and (iii) a zinc oxide promoter; and (b) from about 0.3 part to about 10 parts of a nonsulfur curative system consisting essentially of (i) a nonsulfur curative selected from the group consisting of di- and tri-functional mercapto compounds and derivatives thereof; and (ii) a metal compound promoter selected from the group consisting of oxides, hydroxides, and carbonates of metals in Groups Ia and IIa and said composition cured from about 140° C. to about 260° C. from about 5 seconds to 10 hours.
 2. The composition of claim 1 wherein the sulfur is present in an amount from 0.1 part to about 5 parts.
 3. The composition of claim 1 wherein the accelerator is present in an amount from about 0.1 part to about 5 parts.
 4. The composition of claim 3 wherein the accelerator is selected from the group consisting of mercaptobenzothiazole, mercaptobenzothiazole derivatives, sulfenamides, thiurams, and dithiocarbamate salts.
 5. The composition of claim 4 wherein the accelerator is mercaptobenzothiazole disulfide.
 6. The composition of claim 1 wherein the zinc oxide promoter is present in an amount from about 0.5 part to about 10 parts.
 7. The composition of claim 1 wherein the nonsulfur curative is present in an amount from about 0.1 part to about 5 parts.
 8. The composition of claim 7 wherein the nonsulfur curative is 2-mercapto-1,3,4-thiadiazole-5-benzoate.
 9. The composition of claim 1 wherein (b) (ii) is selected from the group consisting of magnesium oxide and barium carbonate.
 10. The composition of claim 1 wherein (b) (ii) is present in an amount from about 0.2 part to about 10 parts.
 11. The composition of claim 1 wherein (a) is present in an amount from about 1.7 parts to about 9 parts; and (b) is present in an amount from about 0.5 part to about 7 parts.
 12. The composition of claim 11 wherein (a) (i) is present in an amount from about 0.2 part to about 1 part; (a) (ii) is present in an amount from about 0.5 part to about 3 parts; (a) (iii) is present in an amount from about 1.0 part to about 5 parts; (b) (i) is present in an amount from about 0.2 part to about 2 parts; and (b) (ii) is present in an amount from about 0.3 part to about 5 parts. 